This invention relates to position measuring systems for measuring the relative position of first and second objects, of the type comprising a first graduation provided with at least one first reference mark and mounted to a first graduation carrier, means for connecting the first graduation carrier to the first object, and a first scanning unit mounted to scan the first graduation.
In order to determine a reference position in such position measuring systems, it is a common practice to drive the machine parts or measuring system components which are movable relative to one another from a starting position to a reference position defined by a reference mark. Once the reference mark is scanned, a measuring value indicative of the distance traveled up to the reference mark is stored, or alternately a predetermined measuring value is stored at the reference mark for the reference position. Such a calibrating process is possible with an incremental length or angle measuring system such as that described in West German patent DE-PS No. 19 64 381. This calibration process requires that the objects to be measured be movable relative to one another in an unimpeded fashion. This is because the components of the measuring system are firmly joined with the objects to be measured, and therefore the objects to be measured must be adjusted in position along with the associated components of the measuring system as the associated components of the measuring system are brought to the reference position defined by the reference mark.
West German DE-OS No. 16 73 887 describes a measuring system for a machine, which permits the determination of a reference position in the case of a machine slidepiece and machine bed which are firmly clamped in position. First the slidepiece must be driven into the position that is later to be taken as the reference position, for example with a measuring value equal to zero. The slidepiece is then clamped fast to the machine bed. Thereafter, the scanning plate of the measuring system is moved relative to the scale until a reference mark is scanned. On scanning the reference mark, the electronic counter of the measuring system is set to the value Zero. The machine slidepiece can then be released from the machine bed and the slidepiece moved to the desired position. The position of the reference mark therefore defines the reference position for further working operations.
Calibration processes for determining a reference position defined as a starting position, which occurs before the commencement of working operations proper, are not suitable for use with incremental measuring systems if working operations have already begun. For example, a working process in progress may be interrupted and it may then be necessary to reacquire the reference position. Such an interruption of a working operation in progress, for example in a manipulating automatic unit (generally designated as an industrial robot) can occur for example in the event of a power failure. The robot then remains standing at its momentary position at the time of the power failure and the measuring value referred to its original reference position is lost because of the power failure. This is because the power failure typically interrupts the measuring operation. In order to allow the interrupted working process to continue, the reference position must be regained. As a general rule, it is not always possible to move the robot out of its momentary position back to the original starting position, because for example a tool may be in engagement with a workpiece.